Means for improving frequency response of sound systems



E. T. FLEWLLING MEANS FOR IMPROVING FREQUENCY RESPONSE OF' SOUND SYSTEMS Filed D80. 5, 1949 3 Sheets-Sheet l INVENTOR. EDMUND T. FLEvwELLING BYJ ATTORNEY INVENTR. EDMUND T. FLEWELUNG ATTORNEY E 'r FLEWELLING 2,627,931

MEANS FOR IMPROVING FREQUENCY RESPONSE OF SOUND SYSTEMS 3 Sheets-Sheet 2 F I G. 4

Feb. 10, 1953 Filed Deo. 3, 1949 FIG. 3

Feb. 10, 1953 E. T. FLEWELLING 2,627,931

MEANS FOR IMPROVING FREQUENCY RESPONSE OF SOUND SYSTEMS Filel Dec. :5, 1949 3 Sheets-Sheet I5 INVENTOR. EDMUND T. FLEWELLING BYJW ATTORNEY Patented Feb. 10, 1953 UNITED STATES PATENT OFFICE MEANS FOR IMPR-OVING FREQUENCY RESPONSE OF SOUND SYSTEMS Edmund T. Flewelling, Ashburixham, Mass., as-

signor to Stromberg-Carlson Company, a conporation of New York 19 Claims.

This invention relates to sound systems and more particularly to means for improving the frequency response of sound systems, whether installed in homes, buildings of other types, or combined with a radio receiver or a record play er or both.

Heretofore, many suggestions have been made for the improvement of sound reproduction from records, radio receivers, and the like. Because of transients and intangible factors difficult to analyze, previous systems with which I am familiar have, to a greater or lesser degree, failed of accurate and natural reproduction over an extended frequency range. In particular, because of distortion in one form or another and nonuniformity in the boosting of adjacent bands of frequencies, the lower frequencies have not been .reproduced in a satisfactory manner by the methods of bass boost known to me.

Therefore, it is an object of my present invention to provide a new and improved sound system which faithfully reproduces music and other sounds over an extended frequency range and, in particular, with respect to the lower frequencies.

It is another object of my invention to provide a new and improved sound system for use in the reproduction of sound from sound recordings whether incorporated with a radio tuner in a radio cabinet or with an amplifier, or built into a home or other building.

Still another object of my invention is to improve the frequency response of loudspeakers such as are ordinarily used with radio receivers and record players and especially when installed in school rooms, homes, v'moving picture theaters,

and the like. v

Another object of my invention is to provide a new and improved Soundsystem having broad resonance characteristics,'especially with respect to low frequency response, whereby there result no narrow-band, excessive, bass-boost effects A, further object of my invention is to provide a newand improved sound system for the' reproduction of ,sound from recordings in which a natural-sounding response is obtained and in which extraneous noises, such as needle scratch, for example, arereduced to4 a minimum.

Still another object of my invention is to provide means in asound system to correct for loudi.

speaker distortion and to properly load a loudspeaker or loudspeakers.

Still another object of my invention is to provide a sound system embodying `an audio frequency amplifier and acoustic apparatus which Asectional area' of the column is made approximately equalV to the area of the speaker diaphragm. The column is provided with two longitudinally-spacedyapart openings of substantially thesame area. The loudspeaker is suitably mounted in one of these openings whereby the speaker drives across the aircolumn as distinguished from driving through a horn or similar air column as has been done in the past. The area of each of the openings is preferably sub stantially the same as the area of the speaker diaphragm and the cross sectional area of the i' air column. I'employ an amplifier with negative feedback in order to cancel out undesired effects in the higher frequency regions.

In the preferred form of my amplifier there are utilized at least two loudspeakers in the output or voice coil circuit. Each speaker is fed through a condenser-choke coill circuit in such a manner that one speaker reproduces the higher frequencies and another speaker responds principally to the lower frequencies.v A volume control is connected across the voice coil of each speaker or group of speakers in such a manner as substantially to leave unchanged the frequency response of the associated speaker or'speakers but to vary the volume or loudness of the associtaed speaker or speakers. In this manner compensation is provided for pre-emphasis of high or low frequencies of a phonograph recording and also allows the operator of the system to express his preference for the upper and lower frequency ranges. An overall volume control of conventional type may be employed and is used to control the absolute volume after suitable adjustment of the relative volumes of low and high ranges of frequencies.

Substantial results are obtained with the use of my air column, whether or not the amplifier shown and described herein is employed. However, to match the bass response of my system, it is believed that a conventional amplifier system should have bass boost of the orderof forty to acevgesi forty-live db, and feed into an exponential horn of adequate size. These expensive requirements are obviated by the combination described herein.

Further objects and advantages of my invention will become apparent as the following description proceeds, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 is a diagram illustrating the principles and arrangement or an air columnV embodying the principles of my invention; Fig. 2

is a schematic diagram of an amplifier embodying the principles of my invention and useful in the proper reproduction of sound; Fig. 3 illustrates the application of my invention to a radio and record player; Figs. 4 and 5 illustrate modifications of the air columnwhich are particularly useful when my sound' system is` installed in radio cabinets or the like; and Fig'. 6 illustrates the application of the principles of my invention to a home or other building.

The principles of my invention Will be understood from a consideration of Figs. 1 and 2. With respect to Fig. 1 there is shown a rectangular column I. There are provided spaced-apart openings 2 and 3 in opposite walls of the column.

The adjacent edges of the openings are longif tudinally spaced apart as shown. In opening 2 there is mounted, in any suitable manner, a loudspeaker Il in such a manner that when energized by suitable signals, the speaker d drives across the column l as shown in Fig. 1. may be mounted as shown or reversed so that the speaker is entirely within the column and the large end of the diaphragm faces outwardly. With this arrangement it is seen that the back of the loudspeaker is completely separated from the front ofthe speaker and there is no conflict or confusion betweenout-of-phase return waves'.

I have found that best results' are obtained when the cross-sectional area of the column, the

area oi' the diaphragm of loudspeaker 4 and the f cabinet, may be installed in the walls or floor or" a building, as for `example between the floor jcists in a home, or in a radio cabinet, for example. It will be understood that the walls or joists of a building, or the sides and bottom or top ci' a cabinet may comprise walls of the column itself.

I have found that the separation of the openings 2 and 3 is not critical although the openings should not be directly opposite and I prefer to have the longitudinal distance between adjacent edges of the openings indicated by numeral 5 not exceed the approximate diameter of the opening.

I have found that the lengthof the column is not critical although the lovv frequency response improves as the length is increased. I have found that a column of the order of six to eight feet long gives a substantially pure waveform undistorted response at low levels down to approximately 20 cycles when employed in conjunction with a twelve-inch speaker, i. e., the length of the column is preferably of the order of six times the The speaker diameter of the speaker diaphragm and is preferably not less than four times the diameter of the diaphragm. I have also found that columns as short as six inches to two feet in length prove useful at the higher frequencies.

The material from which the column is made may be. of any material such as cardboard, Wood, etc., although improved results are obtained if sound absorbing material is used, or if the column is lined with sound absorbing material, such as fibrousA material, for example, or if the column is lled with sound absorbing material such as rock wool. The provision of sound insulating material reduces cross-reection within the column and reduces or eliminates sharp resonances therein.

Furthermore, the air column may be of any shape and of any cross-sectional arrangement, although I prefer that the cross section shall not deviate to any great extent from a square. In this connection, however, the matching of acoustical impedances yis important and the impedance match of any section of the column to another section must be gradual for best results. Preferably, the column and the openings must maintain a uniform cross-sectional area and any changes of direction of the column or changes in cross-sectional area should be byv means' of rounded corners and gradualtapers, respectively.

I prefer tol provide openinggin opposite walls of the column, but itk is within the scope of my inventionV to place the respective openings in adjacent walls. or even in the' same wall or side of the column. Both ends ofthe' column may be closed,.or one end of the column may be closed and the other open.

Reference is made'to Fig.Y 2V for illustration and description of a suitable. amplier for use with my sound system.r

In the particular' form illustrated there are shown rst and second stages I6 and' I I of amplication utilizingV tubes of the triode type and an output stageV I2 employing two electron discharge devices or tubes of the screen grid type connected` in push-pull.

The output circuit comprises` a pair of choke coils I 3 and I4 and a pair of capacitors I and I6. Capacitor I5 is connected in one side of the output circuit and choke coil I is connected in the other side. Choke coil I3 and capacitor I are connected in series across the output circuit. There is illustrated the use of four loudspeakers connected in two sets.. The rst set` of loudspeakers I and I8 is connected across a potentiometer I. The resistance portion of potentiometei I9 is connected across chokeV i3 and capacitor I5. Similarly, the second set of loud speakers 2i) and 2 I, either or both of which may be used with air column I in the manner shown by speaker 4 as indicated inFig. l, is connected across the potentiometer 22 which in turn is ocnnected across capacitor I6 and' choke coil I With this arrangement potentiometers i Si and Z2 serve as volume controls for the speakers Il, it, and 2l), 2i, respectively, and enable adjustment of the relative volume levels of the sets of loudspeakers which respond generally to the upper and lower frequency ranges, respectively.

The amplifier' shown in Fig. 2 is provided with negative feedback. The feedback circuit cc.. prises a high pass filter including three capacim tance-resistance sections. Each section ccmprises a capacitor 23 connected in series in one side of the feedback circuit and a resistor 2d connected in shunt on the input side of the capacitor.

I' have found that three sections give satisfactory results if eachcf the capacitors is :0.1 microfarad in'capacity'and each resistor is of 30,000 ohms resistance.

In order to provide overall volume control l have illustrated the use of a variable resistance in the grid circuit of the electron discharge device included in stage Iii of the amplifier.

It is thus seen that I prefer to employ a plurality 'of loudspeakers and each set of speakers is preferably arranged to reproduce principally fre quencies in a range less than the complete response range. Inasmuch as a plurality of speakers is used there may also be used a plurality of columns inasmuch as a column may be provided for each speaker. I have found that commercial loudspeakers provide reasonably satisfactory results.

In Fig. 3 there is illustrated a column la having two portions and 3l bent or folded to 90 degrees with respect to each other in order to fit the column into the rear portion of a radio cabinet 32 of conventional shape and size. In order to provide proper impedance matching thebend in the column la is made in the form of a gradual curve. In this arrangement the upper end of the column is closed by the top of the cabinet but the other end remains open. The loudspeaker 4 drives across the column through an opening 2 in the upper side of the column and the audition opening 3 is in the bottom of the cabinet. With this arrangement the side and bottom walls form part of the column.

Fig. 4 illustrates a folded arrangement of a sound column and Fig. 5 illustrates in some detail the constructional details of one form of a folded air column such as that shown in Fig. e. The side walls, not shown, may be sheets of suitable material and the four horizontally disposed walls 33, 34, 35 and 36 may similarly be formed from sheet material. The inner curve may be provided by rounding a suitable block 3l of wood or the like and securing it in a suitable manner to the inner horizontal walls 34 and 35. The outer curve may be provided by utilizing two suitably shaped inserts 38 and 39 of suitable material fastened in any desired manner in the upper and lower corners as illustrated.

In Fig. 6 there is illustrated a typical installation of my invention in a home. As shown, an air column ld, is formed between joists 43 and 4l ofthe floor 42 of a room in a home. The floor l2 forms the upper wall of the column and the lower wall 43 is formed by suitably securing sheet material to the bottom edges of the joists 4B and 4i. The loudspeaker 4 for providing the low frequency response in the order of 20-400 cycles is suitably mounted at the opening 2 in the lower wall of the column and the audition opening 3 is formed in the floor of the room. While there is shown a full opening 3, it is believed obvious that a suitable grill or a plurality of small openings, or the like, would be provided in an actual installation.

I have illustrated a radio tuner lis and record player 45 disposed in the room illustrated although, of course, any other desired arrangement may be employed. I have found that a loudspeaker 46 of the "tweeter type illustrated as mounted below the radio timer is useful when speech or singing is being reproduced and it is desired to provide ay center of interest by giving the listener the impression that the sound is being emitted from a predetermined point. I have found that desirable results are obtained by 6.. locating other speakers in various locations'about the room. For example, I have shown a third loudspeaker 41 in a corner of the room. By variously placing the speakers a. pleasant diffusion of sound is provided.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects. For example, the column may be filled with insulating material, such as rock wool, for example, in which case it is preferred to place a screen across each opening to maintain the insulating material in place. I, therefore, aim in the appended claims to cover all such changes and modications as fall within the true spirit and scope of my invention.

What I claim is:

l. Means for use in a sound reproduction system for providing a broad-resonance low-frequency response over an extended range of low frequency signals including an elongated structure defining an air column said structure having a substantial length with respect to its lateral dimension, .said structure having two non-over- .lapping openings displaced longitudinally with respect to and on opposite sides of said column and a loudspeaker mounted at one of said openings in such a manner as to drive across said column, the second of said openings comprising a sound emitting port, the cross-sectional area of each of said openings being of substantially the same order of magnitude, and the distance between adjacent edges of said openings not exceeding the approximate effective diameter of said loudspeaker.

`2. The dev-ice of claim l in which one end of said column is closed.

3. The device of claim 1 -in which both ends of said column are closed.

fi. The device of claim 1 in which said column contains sound absorbing material.

5. The device of claim 1 in which the length of said column is of the order of six times the effective diameter of said speaker.

6. The device of claim 1 wherein the length of said column is not less than four times the efiective diameter of said speaker.

'7. The device of claim 1 in which said column is folded.

8. The device of claim 'k in which the folds of said column are uniform and gradual so that said column is of substantially uniform cross-sectional dimensions throughout the entire effective length of said column.

9. The device of claim 1 wherein the second of said openings and the back of said loudspeaker are open to the air with a substantially unobstructed sound wave passage therebetween.

l0. The device of claim 1 and baiile means extending outwardly from said structure for increasing the distance between said two openings, whereby the destructive interference between the front and back waves of said speaker is reduced thereby minimizing phase cancellation.

11. Means for use in a sound reproduction system comprising 2, structure defining an air column having two openings longitudinally displaced on opposite sides of said air column, and means including a sound radiating surface arranged to radiate into one of said openings, the cross-sectional area of said air column and of each of said openings being substantially the same as the area of said radiating surface.

12, 'Ijhedeyiceof claim 11 in which 011e end of said air column isclosed,

13. The deviceof'claim ll in which both ends of said column are closed. Y

14. The device of claim l1 in which saidV air column contains sound absorbing material,

l5. The, device ofclaim llin which the length of said air column is in the order of `s ixtimes the efiectivediameter of said sound radiating,r surface.

15. The deviceofv claim 11 in which the length of said columnis not less than four times the effective diameter of'said soundradiating surface.

1,7. Thedevice of claimll in which the distance between the adjacent edges oi said opening does not exceedthe approximate effective diameterof said loudspeal;er,l

18. In a building including a plurality of w a1ls, a sound system comprising; a loudspeakerY having a sound emitting surface, means forrenelosing an elongated air column having a substantial ratio of length to width within at least one of said walls, said air column having a pair lo nonoverlapping longitudinally displaced openings on opposite sides oi said air column, saidloudspeaker being disposed at one of said openings to drive across said column, the second of said openings comprising a sound emitting port, the crosssectional area of each of said'openings being substantially the same and the distance between the adjacent edges of said openings not exceeding` the approximate effective diameter of said sound emitting surface.

19. In a building including a plurality of Walls, a sound system comprising; a loudspeaker havingr a sound emittingsuriacameans for enclosing an elongated air column having a substantial ratio of length to width Within atleast one of said Walls, said air column havingT a pair oi longitudinally displaced openings on opposite sides oi said air column, said loudspeaker being disposed 8` at one ofy said openings ,ton drive intosaid column, thev second of said openings comprising asound emitting,r port, the cross-sectional area ofY each of said openings, of' said air column anciof said soundemitting surface being approximately the same.

EDMUND T. FEEWELLING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Read et al., The Recording and, Reproduction oi Sound, article inY Radio and Television Nevs,December 1943 (pages 8A-50, 12, v122 and Drisko, Getting the MostOut of a Reiiex-Typo Speaker, article in Audio Engineernia]y July 1948. 

